The present invention relates to an electrophotographic digital copier capable of identifying a particular color existing in a document image and, more particularly, to an electrophotographic digital copier capable of producing a bicolor hard copy by a single image transfer.
An electrophotographic digital copier is extensively used which scans a photoconductive element or similar medium by a light beam issuing from a light source to electrostatically form a latent image and develops the latent image by a toner. This type of copier is operable not only as an ordinary copier but also as an output unit included in a facsimile transceiver or a computer. A current trend in the imaging art is toward bicolor or multicolor recording to meet the increasing demand for color recording and to implement color copiers.
A copier capable of producing a bicolor hard copy is disclosed in, for example, Japanese Patent Laid-Open Publication No. 228269/1989. The copier taught in this Laid-Open Publication produces a bicolor hard copy by repeating a copying sequence twice. In the first copying sequence, the copier scans a document image to identify a particular color, i.e., a first color, produces image data representative of the first color, executes predetermined image processing with such image data, writes the image data on a photoconductive element while binarizing the data, develops the written image, transfers the developed image to a recording medium in the form of a paper sheet, fixes the image on the paper sheet, and then temporarily stacks the paper sheet or copy on a refeed tray. In the second copying sequence, the copier scans the same document again to identify a second color different from the first color, produce image data of the second color, executes predetermined processing with the image data, writes the image data on the photoconductive element while binarizing the image data, develops the written image, refeeds the paper sheet carrying the image of the first color from the refeed tray, and then transfers the image of the second color to the paper sheet.
The problem with the above-described conventional copier is that the period of time necessary for a bicolor image to be formed is several times longer than the period of time necessary for a monocolor image to be formed since the copying process has to be repeated twice. Moreover, since the image reading and image transfer is repeated twice each, it is extremely difficult to cause the images of two different colors into register with each other. The above-mentioned Laid-Open Publication discusses various kinds of systems such as one using an image transfer drum and one forming a plurality of images of different colors on a photoconductive element as prior art. However, none of such systems is capable of automatically separating a particular color existing in a document image and transferring toner images of two colors from a photoconductive element to a paper sheet at the same time, i.e., copying a bicolor document at the same speed as a monocolor document.
Japanese Patent Laid-Open Publication No. 87058/1981 discloses an electrophotographic apparatus of the type forming images of two different colors on a photoconductive drum at the same time by a single rotation of the drum. This Laid-Open Publication, however, does not mention an implementation for identifying a particular color of a document image or an implementation for writing image data of identified colors on a photoconductive element.